This invention relates to tubular forms, and, in particular, to a method for coating a surface of a tubular form.
Tubular forms, such as pipes and tubular film-forming extrudates, are coated for a variety of reasons, for example--to improve the gas-barrier characteristics of the form, or to confer heat-sealing properties on a relatively inert film surface. Such coatings are usually applied by depositing a coating material in flowable condition, for example--as a melt, or as a solution or dispersion of the coating material in a liquid vehicle, on to a surface of the tubular form, and spreading the deposited coating material over that surface by means of an appropriately positioned cooperating spreading member. The deposited coating may be spread directly on the surface of the tubular form by a stationary annular spreading member, suitably in the shape of a closely fitting, abrasive-filled, resilient collar encircling the form. Alternatively, the tubular form may first be collapsed to the lay-flat state, and the deposited coating spread by means of at least one flat member, such as a bar or roller, engaging each of the opposed surfaces of the collapsed tube. The hitherto practised techniques thus involve longitudinal axial movement of the tubular form relative to the spreading member, so that the deposited coating material is spread longitudinally on the form surface, and, in practice, have proved difficult to operate to yield consistently uniform, high quality coatings.
Accordingly, the present invention provides a method of coating an axially moving tubular form comprising depositing a coating medium on a surface of the form, and spreading the deposited medium around at least part of the surface in a direction transverse to the direction of movement of the form.
The invention also provides an apparatus for practicing the method according to the invention by coating an axially moving tubular form comprising means for depositing a coating medium on a surface of the form, and means, cooperable with the form surface in a direction transverse to the direction of movement of the form, to spread the deposited medium around at least part of the surface of the form.
Although tubular forms of various cross-sectional shapes may be coated, the technique of the present invention is particularly suitable for the coating of cylindrical tubular forms--for example, a cylindrical, tubular, thermoplastic, polymeric extrudate from which an oriented film can be produced on inflation and stretching by a conventional "bubble" film-forming process.
Reference in this specification to movement of the tubular form in an "axial" direction indicates movement substantially in the direction of the longitudinal axis of the form, and includes movement in a direction slightly inclined to said longitudinal axis, to accommodate the sway or wander normally experienced in conventional processes for the manufacture of tubular forms. Movement of the tubular form through the coating apparatus may be effected under the influence of gravity, or by conventional moving means, such as cooperating rollers, or endless belts, which engage the external surface of the tubular form to forward the latter in the desired direction.
Although, in general, coatings in flowable condition, for example--a molten polymeric material, are suitable for application by the technique of the present invention, we prefer, for convenience and ease of application, to employ coatings in the form of a solution or dispersion of the coating material in a liquid medium. An aqueous solution or dispersion of the coating material is particularly convenient in terms both of cost of the liquid medium, and of safety in avoiding the explosive and toxicity hazards attendant upon the use of volatile organic solvents.
The transverse spreading technique of the present invention ensures that the applied coating medium is positively spread and smoothed onto the form surface by the polishing action created by the tangential wiping motion of the spreading member relative to the moving form surface. This assists the formation of a uniform coating, and avoids "flooding" of the coating medium at regions of the form exhibiting surface indentations and irregularities.
To improve the spreadability of the coating medium and ensure that it remains uniformly dispersed after spreading on the form surface, properties such as the viscosity and surface tension of the coating medium can be adjusted to a suitable value--for example by inclusion in the coating medium of a conventional viscosity modifier, such as a water-soluble polymer, and/or an appropriate surfactant. The appropriate balance of these characteristics depends, inter alia, on the temperature of the coating environment and the rate at which the applied coating is allowed to dry, and is readily established by simple experimentation. Thus, a relatively viscous coating medium, for example--a medium having a viscosity (measured with a Brookfield Viscometer, spindle No. 6) of the order of 22,500 centipoise (10 rpm) or 5,350 centipoise (100 rpm), can be employed.
The technique of the present invention may be employed in the application of coatings of various kinds, but has proved particularly useful in the production of thermoplastic polymeric films having an antistatic coating on a surface thereof. For example, a coating medium comprising an aqueous solution of a quaternary ammonium compound, such as choline chloride, as antistatic agent may be employed. An amine sulphate prepared from `Ethomeen` T12 (supplied by Armour Hess Chemicals Limited) has also proved of value as an independent antistatic agent, and additionally exhibits surfactant characteristics. Accordingly a combination of choline chloride and `Ethomeen` T12 sulphate in aqueous solution has proved to be a particularly effective antistatic coating medium for use according to the present invention. Other antistatic agents, alone or in combination, may be employed, if desired.
The amount of amine sulphate present as the sole antistatic agent in the solution or dispersion applied to the tubular form depends, inter alia, on the level of antistatic properties required in the treated product, and can be readily determined by simple experimentation. Relatively high concentrations of amine sulphate are suitable, provided that the viscosity of the solution or dispersion is not increased to a level which adversely affects the mobility and spreadability thereof, and are advantageous in that the amount of volatile vehicle, which may adversely affect the form surface during evaporation therefrom, is kept to a minimum. In practice, the amine sulphate is conveniently employed at a concentration of up to about 60% by weight of the solution or dispersion, and is suitably employed at concentrations within a range of from 5 to 55%, preferably from 10 to 50%, by weight of the solution or dispersion. However, if the antistatic influence of the amine sulphate is supplemented by the presence of an additional additive, such as choline chloride, the concentration of the amine sulphate may be reduced to a relatively low value, for example of the order of 0.25 to 2.5%, the total concentration of the amine sulphate and the additional additive being within the aforementioned range of up to 60% by weight of the solution or dispersion. Suitably, the concentration of the amine sulphate, alone or together with a supplementary additive, is selected so as to yield a product, such as a polyolefin film derived from the coated form, exhibiting a surface resistivity, measured at 50% Relative Humidity, not exceeding 10 gigohms, and preferably less than 5.0 gigohms.
In practice, we have observed that adequate surfactant behaviour is achieved by the use of a relatively small amount of a long chain amine sulphate. Such salts are less effective, weight for weight, as antistatic agents than short chain quaternary ammonium compounds, such as choline chloride, and we therefore prefer to employ a solution or dispersion comprising a major proportion of the quaternary ammonium compound and a minor proportion of the amine sulphate. Conveniently, the weight ratio of quaternary ammonium compound to amine sulphate in the solution or dispersion applied to the tubular form is from 2:1 to 50:1, preferably between 15.1 and 30:1. As hereinbefore described, the combined concentration of quaternary ammonium compound and amine is desirably such, for example up to about 60% by weight of the applied solution or dispersion, that the viscosity of the solution or dispersion is not increased beyond a level at which a uniform distribution of the additives on the substrate can be achieved.
The amount of coating medium employed will depend, inter alia, on the application envisaged for the coated tubular form, and on the required characteristics of the coated surface--such as coefficient of friction and/or electrical conductivity, but, in the case of an oriented polyolefin film substrate the coating medium is conveniently applied in an amount which will yield an average dry coat thickness within a range of from 0.0005 to 0.03 .mu.m, preferably from 0.001 to 0.002 .mu.m.
Deposition of the coating medium onto a surface of the tubular form is effected in any convenient manner--for example, by spraying, brushing, by discharge from a suitably positioned manifold, or by pumping in the form of an aerated foam. However, to avoid profile defects, we prefer that the coating medium be deposited on the spreading member, and thence transferred, by direct contact, to the form surface.
The means for spreading the deposited coating medium should be such as will smear and spread the coating medium around the tube surface in a direction transverse to the direction of movement of the form. For example, if the coating medium is to be spread on the inside surface of the tube the spreading member may be a disc or mop capable of rotation about the longitudinal axis of the tubular form, and in engagement with the internal surface thereof. In general, however, to facilitate application of the coating medium we prefer to coat the external surface of the tubular form, in which case the spreading member is conveniently an endless belt driven in engagement with the external surface of the tubular form. The belt may encircle the tubular form in such a manner as to spread the deposited coating medium by contact with either the internal or the external surface of the belt. It will be appreciated that a belt assembly of this kind can engage only part of the form surface, and to provide a continuous coating over the entire peripheral surface of the form two or more driven-belt spreaders will be required, the belts being spaced apart along the longitudinal axis of the form, and suitably disposed around the periphery of the form.
In effect, the spreading belt is driven in a plane substantially normal to the longitudinal axis of the moving form, but movement of the latter tends to drag the belt in the direction of movement of the form. As the linear speed of the driven belt is increased, the displacement of the belt by the tubular form becomes progressively less significant, and, desirably, the speed of the belt should be adjusted relative to that of the form to ensure that, as nearly as possible, movement of the belt occurs in a plane normal to the direction of movement of the form. Factors influencing the degree of displacement of the belt include the location of the belt driving means relative to the tubular form, and the nature of the coating medium which, to some extent, acts as a lubricant.
The belt is suitably of a resilient, rubbery material capable of conforming to the surface profile of the tubular form, and is conveniently provided with a ribbed or toothed surface for engagement with an appropriately profiled drive pulley. The opposite surface of the belt, i.e. the contact surface, which engages a surface of the tubular form, is suitably provided with a contact layer of a material capable of spreading and polishing the deposited coating medium to an acceptably uniform finish. The contact material is desirably soft, and inert to the coating medium, and should be selected so as to exhibit an acceptable life span despite the conditions encountered during the coating operation; e.g. the belt may have to operate in a relatively high temperature environment, and is subjected to a constant abrading action against the surface of the tubular form. Lint, or a resilient foam, such as a polyurethane foam, may be employed as the contact layer. Preferably however the contact layer should be of a material which does not absorb the coating medium and therefore retains its resilience and consistency when wetted by the coating medium; neither should it have a porous structure such that the coating medium will dry out to form a hard
By a "self-supporting" film is meant a film capable of independent existence in the absence of a supporting substrate, a polyolefin packaging film being a typical example thereof. Suitable thermoplastic film-forming polymeric materials include polycarbonates, polysulphones, polyamides such as polyhexamethylene adipamide or polycaprolactam, polyesters such as polyethylene terephthalate and polyethylene-1,2-diphenoxyethane-4,4'-dicarboxylate, vinyl polymers and copolymers, and polymers and copolymers of 1-olefins such as ethylene, propylene, butene-1,4-methylpentene-1. A preferred material is a high molecular weight stereoregular predominantly crystalline polymer of propylene, either in the form of a homopolymer or copolymerised with minor quantities (e.g. up to 15% by weight of the copolymer) of other unsaturated monomers, such as ethylene.
An oriented tubular film is suitably produced by melt extruding the desired polymeric material in tubular form from a simple annular die, cooling the extruded tube, reheating and inflating the tube by the so-called "bubble" process to introduce transverse orientation, and simultaneously elongating the tube longitudinally to orient the film in a lengthwise direction. The film is then preferably "heat-set", i.e. dimensional stability of the film is improved by heating the film, while restrained against thermal shrinkage, to a temperature above the glass transition temperature of the polymer from which the film is formed but below the melting point thereof.
A similar technique employing a multi-channel, annular, coextrusion die is suitable for the production of multiple-layer films, such as a polypropylene substrate having on at least one surface thereof a layer of a copolymer of propylene (80 to 95% by weight) with another alpha-olefin containing from 4 to 10 carbon atoms, such as butene-1.
In the production of a coated film according to the invention, the coating medium is conveniently deposited and spread on a surface of the cast, unoriented tubular extrudate immediately prior to the reheating and orienting stage of the film-forming process. Drying of the coating, for example--by evaporation of the volatile solvent or dispersant, is therefore effected during the reheating operation, and the dried coating layer becomes firmly bound to the film surface during orientation.
Coated films made according to the present invention may contain any of the additives conventionally employed in the production of thermoplastic films, and may be subjected to conventional after-treatments--for example, exposure to a corona discharge treatment to improve the bonding and print-receptive characteristics of the film surface.
Films made according to the present invention may vary in thickness depending on the intended application, but usually we find that films having a thickness of from 2 to 150 microns are of general utility. Films intended for use in packaging operations are suitably within a thickness range from 10 to 50 microns.